Differential pressure responsive diaphragm device



Sept. 4, 1956 000K 2,761,471

DIFFERENTIAL PRESSURE RESPONSIVE DIAPHRAGM DEVICE Filed Sept. 20, 1954 2Sheets-Sheet 1 W. 1,, IE; 34 u Z0 22 ENE 42 44 I 46 INVENTQR man CO 0 BYI ATTQR S 2 Sheeis-Sheet 2 L. COOK 8 sa l/ Sept. 4, 1956 DIFFERENTIALPRESSURE RESPONSIVE DIAPHRAGM DEVICE Filed Sept. 20, 1954 surface.

DIFFERENTIAL PRESSURE RESPONSIVE DIAPHRAGM DEVICE Lyman Cook, 'Foxboro,Mass, assignor to The Foxboro Clpmpany, Foxboro, Mass, a corporation ofMassac usetts Application September 20, 1954, Serial No. 457,120

6 Claims. (Cl. 137-788) This invention relates to the measurement ofdifferential pressure and more particularly 'to-a novel differentialpressure responsive diaphragm assembly adapted to be used indifferential pressure measuring instruments'which may indicate, record,or control the differential pressure, or transmit the value of measureddifferential pressure to another instrument or regulating devicelocatedat a remote point. Typical differential pressure measuringinstruments of the kind with which the present diaphragm assembly can beadvantageously used are disclosed'in Cook Patent 2,539,892 and Bowditchapplication "Serial No. 453,632 filed September 1, 1954.

Measuring instruments of the type shown in the 'Cook patent and Bowditchapplication comprise a two-part casing having a resilient corrugatedmetal diaphragm clamped therebetween. The diaphragm divides the interiorof the easing into two chambers which communicate respectively with thetwo pressures between which the difference is to be measured. Theresultant force 'on the resilient diaphragm is transmitted by a forcebar or rod through a flexible portion of the casing wall and thetransmitted force is measured externally of the casing'by a pneumaticforce balancing unit.

In the industrial operation of such instruments it occasionally happensthat one or the other of the Chainbers within the casing is ventedtoatmosphere, either through inadvertence or by design, therebyestablishing a pressure diiference across the diaphragm much larger thanthat which the instrument is intended to measure.

'In order to avoid excessive-deflection and permanent dist-ortion of thediaphragm under such circumstances it is necessary to provide some formof reinforcement which .will :limit the deflection of the diaphragm toits normal operating .range and support the diaphragm during the .periodwhen it is subjected to such over-range pressures.

.In the Cook patent and Bowditch applicationvreferred to above thisobjective is achieved by providing the portions of thelcasing thatconfront the diaphragm withconvolutions that correspond with thecorrugations of'the diaphragm and establishing a spacing between .theconvoluted surfaces of the casing parts that permits a desired range ofdeflection of the diaphragm but prevents exces- While such convolutedsupporting surfaces 'on the :two

sides of the diaphragm should theoretically eliminate dis' they conformprecisely with the corrugations of the dia phragm. Thus when thediaphragm is forcedagain's't one of the convoluted surfaces by a highover-rangepressnre, it is deformed by the small amount that is necessaryto cause it to fit tightly against the convoluted supporting Ifthereafter the diaphragm is subjected to 'a high over-rangepressure onits opposite side, it is forced thereby to conform to'the' convolutionsof the other con- 2,761,471 Patented Sept. 4, 1956 an 5 1C volutedsupporting surface and is thereby again deformed a small am unt.

It has further been found important in minimizing distortion of thediaphragm that the outermost convolution of the supporting surface beconcave and not con- Vex, so that the outermost corrugation of thediaphragm fits into it and the peripherial portion of the diaphragm istherebyniaintained in tension under over-range conditions. It is evidentthat with the structure shown in Cook Patent 2,539,892 wherein thediaphragm is mounted between two supporting surfaces the outermostconvolution of at least one of the surfaces must be convex.

While the amount of diaphragm distortion that occurs under theconditions outlined above is quite small, it is a significant factor inthe operation of instruments of the type referred to'herein. As pointedout above, the force-bar that transmits the force exerted by thediaphragm through the casing wall cooperates at its external end with apneumatic balancing system. The operation of the pnejumatic systemis'such that the normal range of movement of the force baris no morethan a few thousandths of an inch, and hence a very small amount ofdiaphragm distortion is suificient to introduce a significant error inmeasurement.

'In theme of 'suchinstrument for measuring the differential pressure ofliquids, an undesirable pumping action may occur. This is especiallytrue when the pneumatic force'balance mechanism is made highlysensitive. The pumping "action is caused by the reaction of thepneumatic force balance mechanism producing a feed-back motion of thediaphragm which will displace momentarily the column of liquid in thepressure transmitting pipes connecting the instrument to the process.This pumping action makes it difficult, if not impossible,'to interpretthe measurement accurately. When these instruments are used to measuredynamic pulsating differential pressures, as for example those producedby a reciprocating pump, it is similarly difiicult, if'not impossible,to interpret the measurement accurately. In addition, sueh pulsatingdifferential pressures will materially shorten the serviceable life ofthe diaphragm andassociated parts.

It is accordingly an object of the present'invention-to provide animproved diaphragm assembly for use in differential pressure-measuringinstruments. Itis another object of the invention to provide a diaphragmassembly for this purpose which gives effective support to the diaphragmunder over-range pressure conditions and minimizes diaphragm distortionunder these conditions. It is still another object of the invention toprovide a diaphragm assembly including a pressure-responsive diaphragmthat returns precisely-to neutral or Zero position after it has beensubjected to substantial over-range press'ur'es. It is still anotherobject of this invention to provide a diaphragm assembly which, incombination with a higlilys'ensitive' pneumatic force balancemechanisznused to measure the differential pressure of liquids, will produce astable measurement with the wear of the parts of the mechanism reducedto a minimum. Other objects of the invention'will be in part obvious andin part pointed out hereafter.

The many'objects and advantages of the present invention can bestbe'un'derstood and appreciated by reference to the accompanying drawingswhich illustrate a diaphragm assembly incorporating a preferredembodiment of the present invention and wherein:

Figure 1 is a vertical central section ,through the lower portion of thecasing of a force-balanced difierential pres- .sure measuring instrumentand shows a diaphragm as sembly according to the present inventionmounted therein. (In the several views of the drawings the distancesbetween the back-up plate and the diaphragnis are exaggerated toindicate more clearly the manner in which the structure operates.)

Figure 2 is a side elevation of the diaphragm assembly as viewed fromthe plane 22 of Figure 1;

Figure 3 is an enlarged fragmentary vertical section taken on the line3-3 of Figure 2 and showing the positions of the back-up plate anddiaphragms under normal operating conditions;

Figure 4 is a view similar to Figure 3 but showing the position of theparts when subjected to over-range pressures; and

Figure 5 is a fragmentary vertical section through the assembly, also onan enlarged scale, taken on the line 55 of Figure 2 and showing detailsof the means for interconnecting the two diaphragms.

Referring to the drawings and more particularly to Figure 1, the numeralgenerally designates the lower casing of a differential pressuretransmitter of the type disclosed in Bowditch application Serial No.453,632, referred to above. The casing 10 comprises a body 12 and cover14 that are bolted together by suitable bolts (not shown) and haveclamped therebetween a diaphragm assembly 16. The assembly 16 comprisesin general a back-up plate 18 having two corrugated resilient metaldiaphragms 20 and 22 secured at their peripheries to opposite sides ofthe back-up plate.

The diaphragm assembly 16 divides the interior of casing 10 into twochambers 24 and 26, one of which communicates with each of the twopressures of which the differential is to be measured. Thus the externalsurface of diaphragm 20 is exposed to the pressure in chamber 24 and theexternal surface of diaphragm 22 is exposed to the pressure in chamber26.

Referring to Figure 2 as well as to Figure 1, the two diaphragms 20 and22 are of generally circular configuration with circular corrugationsand are interconnected at their centers by a connection 28 which isdescribed in detail hereafter and which passes freely through a centralopening 30 in the back-up plate 18. Thus, when a pressure difference isestablished between the chambers 24 and 26, the diaphragms 20 and 22 andthe connection 28 tend to move toward the lower pressure chamber.Movement of the diaphragm is opposed by a force bar 32 which is mountedin a flexible diaphragm 34 forming part of the wall of the casing and issecured to the connection 28 by a link 36.

The mounting of the force bar 32 in casing 10 and its connection to thediaphragm assembly are more fully described in Bowditch applicationSerial No. 453,632 referred to above. It is also pointed out in theBowditch application that the external end of force bar 32 cooperateswith a pneumaticbalancing system that operates to generate a pressureopposing the movement of the external end of the force bar in suchmanner that the differential force applied to diaphragm assembly 16 isconverted into a corresponding pneumatic pressure with only a very smallmovement of the diaphragms 20 and 22 and the force bar 32. 7

Referring now to Figures 3 and 4 of the drawings, back-up plate 18 has asurface 38 that is provided with convolutions that match thecorrugations of the diaphragm 20. In like manner, the opposite surface40 of back-up plate 18 is provided with convolutions that match thecorrugations of the diaphragm 22. It is desirable that the diaphragms 20and 22 be pressed against the'surfaces 38 and 40 respectively under highpressure prior to final adjustment of the instrument so that thediaphragm corrugations and surface convolutions on the back-up platesurfaces will match precisely. Since each of the diaphragms 20 and 22has only a single back-up surface to which it must conform, thedifiiculties outlined above in connection with structures where a singlediaphragm is positioned between two back-up surfaces are avoided by thepresent structure.

The back-up plate 18 is provided with a relatively narrow peripheralridge 42 over which the peripheral portion of the diaphragm 20 is formedand to which the diaphragm is secured in any suitable manner such as bywelding, brazing, soldering or the like. In like manner the diaphragm 22is secured to a peripheral ridge 44 on the opposite side of back-upplate 18. It will be noted that the outermost corrugation of eachdiaphragm 20 and 22 is convex inwardly and thus the peripheral portionof each diaphragm is under tension when it is forced against the matingsurface of the back-up plate.

The back-up plate may be made relatively thick, to avoid distortion whensubject to overrange pressures. However, when the thickness of theback-up plate is reduced to a minimum practical dimension as a means ofreducing its weight, the manner of mounting of the diaphragm assembly inthe casing has been found to have an important bearing on its properoperation. Thus it has been found that if the diaphragm assembly isrigidly clamped between the casing parts and the assembly is thensubjected to heavy over-range pressures, the central portion of theback-up plate is deflected somewhat and does not return precisely to itsoriginal position, due to a shift of the clamped portion of thediaphragm assembly relative to the casing parts. After the over-rangepressure has been removed, the back-up plate is held deflected in a newposition because of the clamping friction, resulting in a displacementof the mid-point between the diaphragms at zero diflerential pressure,and thereby introducing an error into the measurement.

To overcome this problem the present diaphragm assembly has a flexiblemounting. As shown in Figures 3 and 4, the periphery of the back-upplate 18 is spaced slightly from the adjacent walls of the casing andthe periphery of the back-up plate is provided with an outwardlyextending ring 46 which is of trapezoidal cross-section, that is, it hasa relatively narrow neck portion at the point where it merges into themain body of the back-up plate. The diaphragm assembly is mounted in thecasing by clamping the ring 46 between the casing parts 12 and 14. Thuswhen the diaphragm assembly is subjected to an over range pressure, itflexes at the neck portion 48 as particularly shown in Figure 4 and theback-up plate 18 assumes a bow-shaped configuration. It has been foundthat with this construction when the over-range pressure is removed theback-up plate 18 returns precisely to its original position and hencethere is no displacement of the zero position of the diaphragm assembly.

As pointed out above, the diaphragms 20 and 22 are interconnected by aconnection 28 that passes through an opening 30 in the back-up plate,and the details of this connection are best shown in Figure 5 of thedrawings. Referring to Figure 5, the diaphragms are held in fixedrelation at their centers by a bolt 50 that passes in sequence through awasher 52 which bears against the external surface of diaphragm 20, acentral hole in diaphragm 20, a bushing 54 which bears against the innersurface of diaphragm 20, a second and similar bushing 56 that bearsagainst the inner surface of diaphragm 22, a central hole in diaphragm22, and a washer 58 that bears against the external surface of diaphragm22. The end of bolt 50 is threaded to receive a nut 60 thatis tightenedto hold the washers, bushings and diaphragms 20 and 22 in fixedrelation.

In a diaphragm assembly of this type it is important that the pressureagainst one diaphragm be applied against the other diaphragm in aprecise manner in order to maintain the highest degree of accuracy ofthe measurement. To accomplish this, the free space within the diaphragmassembly is completely filled with a non-compressible liquid. Thus thepressure against one diaphragm is transmitted by the liquid to the otherdiaphragm in a precise manner. In such a liquid filled diaphragmassembly, changes in the temperature of the assembly may cause thevolume of the liquid to change. Any deleterious efof this; changeof'volume of the liquid are substantially eliminated by reducing thefree space within; the

municates with a cross-bore 64 extending transversely through the bolt.The cross-bore 64 communicates with an: annular passage 66 formed in thebushings 54 and 56, and passage 66 in turn communicates through slots68, also. formed in the bushings 54 and 56, with the interior of thediaphragm assembly. Thus liquid can be introducedthrough the end of thebolt 50, passages 62 and 64,

annular passage 66, and slot 68 into the. interior of thediaphragmassembly.

In filling the diaphragm assembly, its interior is first evacuated andthe liquid then introduced so that the liquid will completely fill theinternal space in the assembly. Thereafter a ball 7% is forced into theend of bore 62 and a second closure ball 72 is seated in a. counterbore74 of bolt 50 and sealed in place by any suitable means such as Weldingor soldering or peening the end of the bolt.

In order to prevent leakage of the liquids in the diaphragm assembly,the surface of bushing 54 that. bears against diaphragm 20 is, providedwith a conical recess. 76 containing the packing 78. Bushing 56 has asimilar recess 80 containing the packing 82.

As indicated above, the head of bolt 50 is rigidly secured to a link 36(see Figure 1) which in turn is connected to the. lower end of force-bar32 in such. manner that when a differential pressure is applied acrossthe diaphragm assembly the resultant force. is transmitted to theforce-bar 32.

Instruments of the class of this invention, may be stabilized byattaching a dashpot to the force bar. Such a dashpot will overcome thepumping action referred to previously. However the addition of a dashpotto the external portion of the force bar will not solve the problem ofthe wear of the parts when the instrument is used to measure dynamicpulsating difierential pressures. The liquid filled diaphragm assemblymay advantageously serve as a dashpot as well as a differential pressuresensing means, eliminating the expense of an externally mounted dashpot.In using the liquid filled diaphragm assembly as a dashpot, all movementof parts is reduced to minimum when the instrument is used to measuredynamically pulsating differential pressures and hence the expecteduseful life of the instrument is materially increased. The liquid filleddiaphragm assembly is made to serve as a dashpot by providing arestricted flow of the filling liquid from the free space on one side tothe free space on the other side of the back-up plate 18. This flow ofthe filling liquid may be regulated by suitably proportioning the freespace between central opening 39 in the back-up plate 18 and connection28.

From the foregoing description it should be apparent that the presentinvention provides a structure capable of achieving the several objectsset forth at the beginning of the present specification. The arrangementof the two corrugated diaphragms on opposite sides of a back-up platehaving surface convolutions conforming with the corrugations of thediaphragms eifectively eliminates distortion of the diaphragms, bothbecause they are not required to conform with two different supportingsurfaces and also because the outermost corrugation of each diaphragmcan be made to seat in a recess in the supporting plate and therebymaintain the peripheral portion of each diaphragm in tension when it issubjected to over-range pressure. Moreover, the flexible mounting of theentire assembly permits the back-up plate to deflect under overrangeconditons. and at the same time insures; its return to its preciseoriginal position, thereby eliminating; the errors that are introducedwhen the diaphragm assembly isrigidly clamped at its peripherybetweenthe parts of the casing. In addition, the liquid filleddiaphragnrassembly serves effectively as a dashpot. so that a. highlysensitive pneumatic force balance'mechanism may beused to advantage in.this type of instrument for the measure? ment. of the differentialpressure of liquids and so thatthe instrument may be used satisfactorilyto measure: dynamic pulsating dilferential pressures.

It is of course to be understood that the foregoing description isillustrative only and that numerous changes can be made in theparticular embodiment described without. departing. from the spirit ofthe invention as. set forth in the appended claims.

I claim:

1. In differential pressure responsive, apparatus in com.- bination, aback-up plate, a pair of corrugated metal. diaphragms secured at theirperipheries to opposite sides of saidzplate to form a hermeticallysealed unit, each of said diaphragms, when said unit is subjected toequal pressures on opposite sides thereof, being slightly spaced fromthe adjacent side of said back-up plate throughout the effective area.of said plate to permit a limited amount of inward deflection of saiddiaphragm in response to externally appliedpressure without altering theeffective area of said diaphragm, the opposite sides of said back-upplate being provided with convolutions thatconform with the diaphragmcorrugations whereby when excessively high pres.- sures are applied tooneof said diaphragms, the diaphragm seats. snugly against said back-upplate and the plate prevents, distortion of said diaphragm, said back-upplate having apassage therethrough establishing communication betweensaid diaphragms,, and means for applying fluid pressures to oppositesides of said sealed unit. to establish a differential pressurethereacross.

2. In differential pressure responsive apparatus incombination, aback-up plate, a pair of corrugated metal diaphragms secured at theirperipheries to opposite sides of said plate to form a hermeticallysealed unit, each of said diaphragms, when said unit is subjected toequal pressures on opposite sides thereof, being slightly spaced from.the adjacent side of saidback-up plate throughout theeifective area ofsaid plate to permit a limited amount of inward reflection of saiddiaphragm in response to externally applied pressure without alteringthe effective area of said diaphragm, the opposite sides of said back-upplate being provided with convolutions that conform with the diaphragmcorrugations whereby when excessively high pressures are applied to oneof said diaphragms, the diaphragm seats snugly against said back-upplate and the plate prevents distortion of said diaphragm, said back-upplate having a passage therethrough opposite the centers of saiddiaphragms for establishing communication between said diaphragms, amechanical connection extending through said passage and connected atits opposite ends to said diaphragms, and means for applying fluidpressures to opposite sides of said sealed unit to establish adifferential pressure thereacross.

3. In differential pressure responsive apparatus in combination, aback-up plate having a series of radially spaced circular channels ineach side thereof, a pair of metal diaphragms having circularcorrugations constructed and arranged to fit into the channels of saidback-up plate, said diaphragms being secured at their peripheries toopposite sides of said plate to form a hermetically sealed unit, each ofsaid diaphragms, when said unit is subjected to equal pressures onopposite sides thereof, being slightly spaced from the adjacent side ofsaid back-up plate throughout the effective area of said plate to permita limited amount of inward deflection of said diaphragm in response toexternally applied pressure without altering the effective area of saiddiaphragm, said back-up plate having a passage therethrough establishingcommunication between said diaphragms and the free space in said sealedunit being substantially completely filled with liquid, and means forapplying fluid pressures to opposite sides of said sealed unit toestablish a difierential pressure thereacross.

4. In differential pressure responsive apparatus in combination, aback-up plate having a series of radially spaced circular channels ineach side thereof, a pair of metal diaphragms having circularcorrugations constructed and arranged to fit into the channels of saidback-up plate, whereby said back-up plate supports and preventsdistortion of said diaphragrns when a large differential pressure isapplied thereto, said diaphragms being secured at their peripheries toopposite sides of said plate to form 3. hermetically sealed unit, eachof said diaphragms, when said unit is subjected to equal pressures onopposite sides thereof, being slightly spaced from the adjacent side ofsaid back-up plate throughout the efliective area of said plate topermit a limited amount of inward deflection of said diaphragm inresponse to externally applied pressure without altering the effectivearea of said diaphragm, said back-up plate having a passage therethroughestablishing communication between said diaphragms, the peripheralcorrugation of each of said diaphragms being convex on the sideconfronting said back-up plate whereby the diaphragm is in tension whenforced against said back-up plate by said large differential pressure,and means for applying fluid pressures to opposite sides of said sealedunit to establish a differential pressure thereacross.

5. In differential pressure responsive apparatus in combination, aback-up plate, a pair of corrugated metal diaphragms secured at theirperipheries to opposite sides of said plate to form a hermeticallysealed unit, each of said diaphragms, when said unit is subjected toequal pressure on opposite sides thereof, being slightly spaced from theadjacent sides of said back-up plate throughout the effective area ofsaid plate to permit a limited amount of inward deflection of saiddiaphragm in response to externally applied pressure without alteringthe efiective area of said diaphragm, the opposite sides of said back-upplate being provided with convolutions that conform with the diaphragmcorrugations, whereby when excessively high pressures are applied to oneof said diaphragms, the diaphragm seats snugly against said back-plateand the plate prevents distortion of said diaphragm, said back-up platephragms secured at their peripheries to opposite sides of said plate toform a hermetically sealed unit, each of said diaphragms, when said unitis subjected to equal pressures on opposite sides thereof, beingslightly spaced from the adjacent sides of said back-up plate throughoutthe effective area of said plate to permit a limited amount of inwarddeflection of said diaphragm in response to externally applied pressurewithout altering the eficctive area of said diaphragm, the oppositesides of said back-up plate being provided with convolutions thatconform with the diaphragm corrugations, whereby when excessively highpressures are applied to one of said diaphragms, the diaphragm seatssnugly against said back-up plate and the plate prevents distortion ofsaid diaphragm, said back-up plate having passage therethroughestablishing communication between said diaphragms, the peripheralcorrugation of each of said diaphragms being convex on the sideconfronting said back-up plate whereby the diaphragm is in tension whenforced against said back-up plate by said excessively high pressures, amechanical connection extending through said passage and interconnectingsaid diaphragms, the free space in said sealed unit being substantiallycompletely filled with liquid, and means for applying fluid pressures toopposite sides of said sealed unit to establish a difierential pressurethereacross.

References Cited in the file of this patent UNITED STATES PATENTS

